It is known to produce sintered workpieces from a mass of fine particles consisting in a major proportion of soft (ductile or malleable) iron and in a minor proportion of one or more ferroalloys, with admixture of elemental carbon (usually in the form of graphite) to the extent required in the steel which results from the sintering process. In some instances, especially in the case of tool or high-speed steels which characteristically contain various carbides, the ferroalloy particles are produced separately from the soft iron as a mixture of comminuted iron carbide with the comminuted carbides of several non-ferrous metals, e.g. chromium, tungsten, vanadium and molybdenum along with titanium, tantalum and/or niobium. Typical compositions of such ferroalloys, in which the aforementioned nonferrous metals as well as iron are present as individual carbides, contain carbon up to 15%, chromium from 5% to 30%, tungsten up to 35%, vanadium up to 25%, molybdenum up to 30%, and up to 15% of each of the other elements referred to, balance iron and the usual metallurgical impurities present in the melt from which these carbides are obtained; the iron, as a rule, amounts to substantially less than 50% (by weight) of the composition. Such a mixture can be comminuted mechanically, i.e. by grinding, or by a hot spray. The ferroalloy carbides retain their identity in the subsequent sintering operation.
For other types of alloy steel, particularly those including manganese, it has been the practice to blend the iron with its nonferrous additives directly in the melt whose composition therefore corresponds to that of the final product. A diadvantage of this method is the unavoidable oxidation of the metals, especially manganese and chromium, the oxygen content of the steel impairing the mechanical properties of the resulting workpieces. Furthermore, the production of workpieces of reasonable strength from such compositions requires the use of compacting pressures which exceed by a factor of 2 the pressure normally employed in the powder metallurgy of iron. Attempts to produce satisfactory alloy steels containing manganese, molybdenum, chromium, vanadium and/or niobium by this technique have been particularly unsuccessful.
It has also been proposed to form a sinterable mass by admixing complex carbides of nonferrous metals, e.g. chromium and manganese, with iron powder. These carbides have a relatively low carbon content and their metallic constituents are prone to oxidation prior to sintering, a process which is not easily reversible even when sintering is carried out in a protective atmosphere including a getter.